Natural gas has increasingly become an attractive alternative to other fuels for use in internal combustion engines. For instance, in one class of engines, a relatively large charge of natural gas is ignited by compression igniting a small pilot quantity of liquid diesel fuel. In one application, dual fuel compression ignition engines are utilized for powering mining trucks. The natural gas may be stored in liquid form at very cold temperatures (e.g., −160° C.) and near atmospheric pressure. As the liquefied natural gas in the storage tank absorbs heat, some of the gas evaporates off. If the evaporated gas is not used fast enough during normal engine operations, the tank can eventually build enough pressure that it needs to vent the excess evaporated gaseous fuel. Since natural gas is primarily methane, venting the natural gas to atmosphere is considered very undesirable, as methane is known as an extremely powerful greenhouse gas. In practice, liquefied natural gas tanks are often constructed to vent at around 200 PSI. In order to be commercially viable, an engine system that utilizes natural gas might need to have some strategy for limiting the amount of natural gas, and especially methane, that is vented to atmosphere.
U.S. Pat. No. 6,698,211 teaches a natural gas fuel storage and supply system for gas burning engines of a vehicle. This reference teaches a passive strategy that routes evaporated natural gas from the storage tank to the intake manifold of the engine while the engine is in operation. This reference also teaches inclusion of a restriction orifice that limits the flow rate of evaporated natural gas so that the air/gas mixture traveling through the intake manifold is sufficiently lean to avoid, or limit the risk of, combustion of the air/gas mixture in the intake manifold. This reference also teaches that a flow control valve may be substituted in place of the restricting orifice. Although this reference recognizes a desirability to burn rather than vent evaporated natural gas from a storage tank, it fails to contemplate other problems that could arise in such systems, especially dual fuel compression ignition engine systems.
Dual fuel engines that utilize natural gas typically have some volume of pressurized natural gas available at all times during normal engine operation. At time of engine shut down, disposing of an existing surplus volume of pressurized gas can potentially be a concern.
The present disclosure is directed toward one or more of the problems set forth above.